Abstract. We conducted a comprehensive investigation on the microzooplankton herbivory
effect on phytoplankton in the northern South China Sea (SCS) using the
seawater dilution technique at surface and deep chlorophyll maximum (DCM)
layers on two cruises (July–August of 2009 and January of 2010). We compared
vertical (surface vs. DCM), spatial (onshore vs. offshore), and seasonal
(summer vs. winter) differences of phytoplankton growth (μ0) and
microzooplankton grazing rates (m). During summer, both μ0 and
m were significantly higher at the surface than at the DCM layer, which
was below the mixed layer. During winter, surface μ0 was
significantly higher than at the DCM, while m was not significantly different
between the two layers, both of which were within the mixed layer. Surface
μ0 was, on average, significantly higher in summer than in winter,
while average surface m was not different between the two seasons. There were
no cross-shelf gradients of μ0 in summer or winter surface
waters. In surface waters, μ0 was not correlated with ambient
nitrate concentrations, and the effect of nutrient enrichment on
phytoplankton growth was not pronounced. There was a decreasing trend of m
from shelf to basin surface waters in summer, but not in winter.
Microzooplankton grazing effect on phytoplankton (m/μ0) was
relatively small in the summer basin waters, indicating a decoupling of
microzooplankton grazing and phytoplankton growth at this time. On
average, microzooplankton grazed 73% and 65% of the daily primary
production in summer and winter, respectively.